The present invention is directed to a circuit arrangement having a power MOSFET and an inductive load that is connected, first, to the source terminal of the MOSFET and, second, to a terminal having a fixed potential, and a series circuit composed of a Zener diode and of a controllable switch connected between the gate terminal of the MOSFET and the terminal.
For example, such a circuit arrangement, has been disclosed by European Patent No. 0 239 861. For an activated inductive load, deactivating of the inductive load is initiated by turning on the controllable switch. The latter discharges the gate-source capacitance of the power MOSFET that thus begins to turn off. The current through the inductive load initially continues to flow and builds up a voltage in non-conducting direction at the Zener diode that can reach the Zener voltage. The driving voltage at the inductive load is therefore the Zener voltage plus the voltage drop across the controllable switch plus the gate-source voltage at the power FET. The driving voltage at the inductive load can therefore be essentially set on the basis of the selection of the Zener voltage. The magnetic energy is then quickly reduced for a high Zener voltage.
When the power MOSFET is operated by an electronic switch, its gate terminal lies at 0 volts in a turned-off condition. When the voltage at the inductive load is reversed, the power MOSFET can begin to switch on when its emitter potential has fallen below zero by the threshold voltage. The driving voltage at the inductive load is therefore limited to a value that corresponds to the threshold voltage of the power MOSFET. The magnetic energy in this case is therefore only slowly reduced.